Note: These examples put the carry *underneath* the two subtrahends.
The "Carry" flag is used as a "borrow" flag for subtraction.

            0 1 0 1   0 1 1 0   1 0 0 0   1 1 0 1 first number
          - 0 0 1 1   0 0 0 1   1 1 0 0   1 1 1 0 second number
(-borrow) 0 0 1 0 0   0 0 1 1   1 1 1 1   1 1 0   <== borrow
          _______________________________________
            0 0 1 0   0 1 0 0   1 0 1 1   1 1 1 1 ANSWER
Zero:off  
Carry:off  
Sign:off
Overflow:off (subtraction of two like-signed values is always valid in 2's Complement)

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            1 0 0 0   1 1 1 1   0 0 1 1   1 0 0 1 first number
          - 0 1 1 1   0 0 0 0   1 1 0 0   0 1 1 1 second number
(-borrow) 0 1 1 1 0   0 0 0 1   1 0 0 0   1 1 0   <== borrow
          ________________________________________
            0 0 0 1   1 1 1 0   0 1 1 1   0 0 1 0  ANSWER
Zero:off
Carry:off
Sign:off
Overflow:on (negative minus a positive is like adding two negatives and so
             the answer should be negative; if not, Overflow goes ON)


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